The present invention relates to the preparation of particulates of so-called Magneli phase suboxides of titanium and their use as electrodes and substrates for electrodes including conductivity aids in corrosive environments. It has been found that such particulates, when made within the limits of this invention, have utility in electrical storage batteries and fuel cells, corrosion resistant films, and large industrial metal winning electrodes.
Titanium suboxides having the general formula TiO.sub.x where x is a number in the region 1.55 to 1.95 are taught as electrode materials in various electrochemical settings in U.S. Pat. No. 4,422,917. This patent discloses that powders of TiO.sub.x, where x is in the region of 1.55 to 1.95, might be used as conducting backfill for ground bed electrodes, or where coated, might be used in fluidized bed-type electrodes. TiO.sub.x, where x is 1.75, is identified as the most conductive composition between TiO and TiO.sub.2.
Japanese patent 61-106414 describes the preparation of TiO.sub.x powders, where x is 1.5-1.9, and discloses that such material may be used as "conductivity imparting agents" for supports to recording materials such as anti-static agents, or as pigment for plastics, paints, inks and cosmetics. No mention is made of electrochemical applications which depend on corrosion resistance and chemical stability.
TiO.sub.x particulate materials made from the suboxides described in U.S. Pat. No. 4,422,917 have a much higher electrical resistance in air than the bulk ceramic. For example, powders made by the teachings of U.S. Pat. No. 4,422,917 have resistances of 10.sup.6 times the resistance of the bulk ceramic made under identical conditions. On the other hand, this very high resistance is in direct contrast to the materials disclosed in Japanese patent 61-106414 which are much more conductive even in a much finer particle size.
While these apparently similar materials have widely varying conductive properties, titanium suboxide particulates prepared from the materials described in U.S. Pat. No. 4,422,917 and Japanese patent 61-106414 are both chemically unstable in corrosive environments such as typical electrolytes, and release soluble titanium ions in such solutions.
At least two problems arise from the presence of oxides that form soluble titanium salts from bulk ceramic and particulate suboxides of titanium with respect to their use in electrochemical processes. First the loss of any materials from the surface and intergranular boundary of the bulk ceramic weakens the structure and may dislodge catalysts which are often applied to the surface thereof. Soluble titanium ions may hydrolyze and oxidize to form nonconductive titania at the surface, isolating the base material electrochemically.
A second problem is the presence of soluble titanium ions in the electrolyte solution which may interfere with the electrochemical reactions taking place at the anode and/or cathode. Titanium ions are capable of acting as a redox reagent in electrochemical cells, being oxidized and reduced at the anode and cathode respectively. Their presence in batteries for example would be deleterious to the activity of the battery. So it is with many electrochemical applications in which conductive titanium suboxides might be used. The presence of soluble or insoluble corrosion products can negate the advantages of such a material.
The anamolous resistive properties and solubility problems associated with particulate suboxides have, up to now, been of relatively minor importance, since titanium suboxides have generally been used in bulk, coherent, ceramic form as electrodes. As potential uses for particulate forms are identified however, these problems have proven to be a substantial barrier to consistent performance.
In light of the many potential advantageous uses of conductive titanium suboxide particulates in electrochemical settings, it is an object of the present invention to identify and produce a titanium suboxide composition which, as a particulate material, is not only satisfactorily conductive, but also exhibits long-term resistance to degradation in electrolytes commonly used in electrochemical processes.